Fibers crosslinked in substantially individualized form and various methods for making such fibers have been described in the art. The term "individualized, crosslinked fibers", refers to cellulosic fibers that have primarily intrafiber chemical crosslink bonds. That is, the crosslink bonds are primarily between cellulose molecules of a single fiber, rather than between cellulose molecules of separate fibers. Individualized, crosslinked fibers are generally regarded as being useful in absorbent product applications. The fibers themselves and absorbent structures containing individualized, crosslinked fibers generally exhibit an improvement in at least one significant absorbency property relative to conventional, uncrosslinked fibers. Often, the improvement in absorbency is reported in terms of absorbent capacity. Additionally, absorbent structures made from individualized crosslinked fibers generally exhibit increased wet resilience and increased dry resilience relative to absorbent structures made from uncrosslinked fibers. The term "resilience" shall hereinafter refer to the ability of pads made from cellulosic fibers to return toward an expanded original state upon release of a compressional force. Dry resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a substantially dry condition. Wet resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a moistened condition. For the purposes of this invention and consistency of disclosure, wet resilience shall be observed and reported for an absorbent structure moistened to saturation.
In general, three categories of processes have been reported for making individualized, crosslinked fibers. These processes, described below, are herein referred to as dry crosslinking processes, aqueous solution crosslinking processes, and substantially non-aqueous solution crosslinking processes.
Processes for making individualized, crosslinked fibers with dry crosslinking technology are described in U.S. Pat. No. 3,224,926, L. J. Bernardin, issued Dec. 21, 1965. Individualized, crosslinked fibers are produced by impregnating swollen fibers in an aqueous solution with crosslinking agent, dewatering and defiberizing the fibers by mechanical action, and drying the fibers at elevated temperature to effect crosslinking while the fibers are in a substantially individual state. The fibers are inherently crosslinked in an unswollen, collapsed state as a result of being dehydrated prior to crosslinking. Processes as exemplified in U.S. Pat. Nos. 3,224,926, wherein crosslinking is caused to occur while the fibers are in an unswollen, collapsed state, are referred to as processes for making "dry crosslinked" fibers. Dry crosslinked fibers are generally highly stiffened by crosslink bonds, and absorbent structures made therefrom exhibit relatively high wet and dry resilience. Dry crosslinked fibers are further characterized by low fluid retention values (FRV).
Processes for producing aqueous solution crosslinked fibers are disclosed, for example, in U.S. Pat. No. 3,241,553, F. H. Steiger, issued Mar. 22, 1966. Individualized, crosslinked fibers are produced by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers produced in this manner are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling effect of water on cellulosic fibers, aqueous solution crosslinked fibers are crosslinked while in an uncollapsed, swollen state. Relative to dry crosslinked fibers, aqueous solution crosslinked fibers as disclosed in U.S. Pat. No. 3,241,553 have greater flexibility and less stiffness, and are characterized by higher fluid retention value (FRV). Absorbent structures made from aqueous solution crosslinked fibers exhibit lower wet and dry resilience than structures made from dry crosslinked fibers.
In U.S. Pat. No, 4,035,147, Sangenis et al., issued Jul. 12, 1977, a method is disclosed for producing individualized, crosslinked fibers by contacting dehydrated, nonswollen fibers with crosslinking agent and catalyst in a substantially nonaqueous solution which contains an insufficient amount of water to cause the fibers to swell. Crosslinking occurs while the fibers are in this substantially nonaqueous solution. This type of process shall hereinafter be referred to as a nonaqueous solution crosslinked process; and the fibers thereby produced shall be referred to as nonaqueous solution crosslinked fibers. The nonaqueous solution crosslinked fibers disclosed in U.S. Pat. No. 4,035,147 do not swell even upon extended contact with solutions known to those skilled in the art as swelling reagents. Like dry crosslinked fibers, they are highly stiffened by crosslink bonds, and absorbent structures made therefrom exhibit relatively high wet and dry resilience.
Crosslinked fibers as described above are believed to be useful for lower density absorbent product applications such as diapers and also higher density absorbent product applications such as catamenials. However, such fibers have not provided sufficient absorbency benefits, in view of their detriments and costs, over conventional fibers to result in significant commercial success. Commercial appeal of crosslinked fibers has also suffered due to safety concerns. The crosslinking agents most widely referred to in the literature are formaldehyde and formaldehyde addition products known as N-methylol agents or N-methylolamides, which, unfortunately, cause irritation to human skin and have been associated with other human safety concerns. Removal of free formaldehyde to sufficiently low levels in the crosslinked product such that irritation to skin and other human safety concerns are avoided has been hindered by both technical and economic barriers.
As mentioned above, the use of formaldehyde and various formaldehyde addition products to crosslink cellulosic fibers is known in the art. See, for example, U.S. Pat. No. 3,224,926, Bernardin, issued on Dec. 21, 1965; U.S. Pat. No. 3,241,553, Steiger, issued on Mar. 22, 1966; U.S. Pat. No. 3,932,209, Chatterjee, issued on Jan. 13, 1976; U.S. Pat. No. 4,035,147, Sangenis et al, issued on Jul. 12, 1977; and U.S. Pat. No. 3,756,913, Wodka, issued on Sep. 4, 1973. Unfortunately, the irritating effect of formaldehyde vapor on the eyes and skin is a marked disadvantage of such references. A need is evident for cellulosic fiber crosslinking agents that do not require formaldehyde or its unstable derivatives.
Other references disclose the use of dialdehyde crosslinking agents. See, for example, U.S. Pat. No. 4,689,118, Makoui et al, issued on Aug. 25, 1987; and U.S. Pat. No. 4,822,453, Dean et al, issued on Apr. 18, 1989. The Dean et al reference discloses absorbent structures containing individualized, crosslinked fibers, wherein the crosslinking agent is selected from the group consisting of C.sub.2 -C.sub.8 dialdehydes, with glutaraldehyde being preferred. These references appear to overcome many of the disadvantages associated with formaldehyde and/or formaldehyde addition products. However, the cost associated with producing fibers crosslinked with dialdehyde crosslinking agents such as glutaraldehyde may be too high to result in significant commercial success. Therefore, there is a need to find cellulosic fiber crosslinking agents which are both safe for use on the human skin and also commercially feasible.
The use of polycarboxylic acids to impart wrinkle resistance to cotton fabrics is known in the art. See, for example, U.S. Pat. No. 3,526,048, Roland et al, issued Sep. 1, 1970; U.S. Pat. No. 2,971,815, Bullock et al, issued Feb. 14, 1961 and U.S. Pat. No. 4,820,307, Welch et al, issued Apr. 11, 1989. These references all pertain to treating cotton textile fabrics with polycarboxylic acids and specific curing catalysts to improve the wrinkle resistance and durability properties of the treated fabrics.
It has now been discovered that ester crosslinks can be imparted onto individualized cellulosic fibers through the use of specific polycarboxylic acid crosslinking agents. The ester crosslink bonds formed by the polycarboxylic acid crosslinking agents are different from the crosslink bonds that result from the mono- and di-aldehyde crosslinking agents, which form acetal crosslinked bonds. Applicants have found that absorbent structures made from these individualized, ester-crosslinked fibers exhibit increased wet resilience and dry resilience and improved responsiveness to wetting relative to structures containing uncrosslinked fibers. Important y, the polycarboxylic acids disclosed for use in the present invention, are nontoxic, unlike formaldehyde and formaldehyde addition products commonly used in the art. Furthermore, the preferred polycarboxylic crosslinking agent i.e., citric acid, is available in large quantities at relatively low prices making it commercially competitive with formaldehyde and formaldehyde addition products, without any of the related human safety concerns.
It is an object of this invention to provide a process for preparing individualized fibers, crosslinked with a polycarboxylic acid crosslinking agent, which have improved absorbency properties. Absorbent structures made from the individualized, polycarboxylic acid crosslinked fibers exhibit higher wet resilience and higher dry resilience than structures made from uncrosslinked fibers.
It is a further object of this invention to provide individualized fibers crosslinked with a polycarboxylic crosslinking agent and absorbent structures made from such fibers, as described above, which have a superior balance of absorbency properties relative to prior known crosslinked fibers.
It is additionally an object of this invention to provide a commercially viable process for preparing individualized, crosslinked fibers and absorbent structures made from such fibers, as described above, which can be safely utilized in the vicinity of human skin.